Pressure-sensitive adhesives (PSAs) are adhesives which even under a relatively weak applied pressure permit a durable connection with the substrate and which after service can be detached from the substrate again substantially without residue. PSAs are permanently pressure-sensitively adhesive at room temperature, thus having a sufficiently low viscosity and a high tack, meaning that they wet the surface of the respective substrate even under low applied pressure. The adhesive bonding capacity of the adhesives and their redetachability derive from their adhesive properties and from their cohesive properties. A variety of compounds are contemplated as a basis for PSAs.
Adhesive tapes furnished with PSAs, referred to as pressure-sensitive adhesive tapes, are presently in diverse use within the industrial and household spheres. Pressure-sensitive adhesive tapes consist customarily of a carrier film furnished on one or both sides with a PSA. There are also pressure-sensitive adhesive tapes which consist exclusively of a PSA layer and no carrier film, these being known as transfer tapes. The composition of the pressure-sensitive adhesive tapes may vary greatly and is dependent on the particular requirements of the various applications. The carriers consist customarily of polymeric films such as polypropylene, polyethylene, or polyester, for example, or else of paper, woven fabric, or nonwoven material. The self-adhesives or PSAs are based generally on acrylate copolymers, silicones, natural rubber, synthetic rubber, styrene block copolymers, or polyurethanes.
There is a need for adhesive tapes which exhibit a very high bonding strength, but also do not lose their cohesion at elevated temperatures. In the case of adhesive bonds in the exterior sector or in automobiles, in particular, temperatures of more than 60° C. to 70° C. may occur. For adhesive tapes with particularly high holding powers, especially in the consumer segment, adhesives based on styrene block copolymers are frequently employed. An advantage of these adhesives is that they are able to exhibit very high bonding strength in tandem with very high cohesion. As a result of a high tack, bonds even to rough substrates are reliably possible.
Typically finding use are linear or radial block copolymers based on polystyrene blocks and polybutadiene blocks and/or polyisoprene blocks, in other words, for example, radial styrene-butadiene (SB)n and/or linear styrene-butadiene-styrene (SBS) and/or linear styrene-isoprene-styrene (SIS) block copolymers.
The products that are on the market with PSAs based on styrene block copolymers exhibit weaknesses in their bonding strength at temperatures above 50° C. Especially when being used for bonding articles of moderate weight, the softening of the hard phases consisting principally of polystyrene (block polystyrene domains) results in cohesive failure of the pressure-sensitive adhesive strips.
Bond failure occurs to a significantly greater extent in particular in the case of a tipping shear load (when a torque is active, such as in a hook bond, for example) than in the case of a pure shear load.
It is an object of the invention to provide an adhesive which exhibits excellent temperature stability. The adhesive is in particular to be a pressure-sensitive adhesive, with a tack across a broad temperature range; more particularly, the adhesive is to be suitable for temperature-stable adhesive tapes. A further object is to develop an adhesive which exhibits good bonding strength at high temperature. Another object is to develop an adhesive which is able to bond different articles durably to one another, which has good performance at high temperatures, more particularly good bonding strength at high temperatures, preferably above 100° C., and which is easy to handle in bonding operations, which permits flexible and clean processing, and which is nevertheless easy for the manufacturer to process.